談起Ajax做過web開發的都非常熟悉,就是通過xmlhttp request與服務器端通信而避免頁面刷新。關於Ajax是如何運作的,網上有很多帖子解釋其各js文件的作用及調用xmlhttp的原理。但Ajax到底是怎麼調用服務器端的C#代碼的呢?怎麼讓後臺的方法運行並將結果反饋給xmlhttp的呢?曾經有個同事問起我這個問題,我還真懵了!本以爲象.Net 1.1下通過form傳遞必要的EventName及EventPara等參數傳給服務器端繼而解析後執行對應的事件一樣來調用C#代碼的(.net調用事件機制也不全是這麼回事,待探討),但通過仔細研究,發現原來遠不是這麼回事,而網上更深入的文章卻少之又少。
我們由淺到深吧,先看看相對錶象的東西,即前臺Ajax相關的JavaScript代碼部分。之所以說相對膚淺和表象,是因爲這些資料很多網友已經撰文解讀過。
凡要使用AjaxPro,我們大致要做以下工作:
1) 在項目中引用AjaxPro.dll(我用的是AjaxPro.2.dll,版本6.6.13.2),並在web.config中httpHandlers配置節添加:
<add verb="POST,GET" path="ajaxpro/*.ashx" type="AjaxPro.AjaxHandlerFactory, AjaxPro.2"/>
2) 在要使用Ajax功能的頁面.cs文件上註冊Ajax,例如:
protected void Page_Load(object sender, EventArgs e)
{
// 註冊Ajax
AjaxPro.Utility.RegisterTypeForAjax(typeof(Default));
}
3) 在.cs文件中聲明可以被Ajax調用的函數(或屬性),如:
[AjaxPro.AjaxMethod]
public string GetChild(string parentId)
{
return "return value from .cs file";
}
4) 在.aspx文件中用JavaScript調用Ajax,如:
<script language="javascript">
var items = DynLoadTree.Default.GetChild( "aa" ).value; // 通過Ajax調用後臺代碼
alert(items);
</script>
做好以上四步,我們就基本實現了Ajax頁面不刷新的功能了。那麼它是怎樣通過xmlhttp與服務器通訊的呢?運行後我們可以看到HTML文件的源代碼多了幾行.ashx文件的引用:
<script type="text/javascript" src="/ajaxpro/prototype.ashx"></script>
<script type="text/javascript" src="/ajaxpro/core.ashx"></script>
<script type="text/javascript" src="/ajaxpro/converter.ashx"></script>
<script type="text/javascript" src="/ajaxpro/DynLoadTree.Default,DynLoadTree.ashx"></script>
實際上這些.ashx就是在上面第2步AjaxPro.Utility.RegisterTypeForAjax註冊Ajax時自動將這些引用添加到Html文檔輸出的。那這些文件是什麼文件呢?再看第1步中在web.config中添加到httpHandlers節中的配置,它告訴系統凡是收到ajaxpro路徑下已經ashx爲後綴的請求就全部交給AjaxPro.AjaxHandlerFactory這個類來處理,而這些ashx經過處理後返回的就是一些JavaScript文件,和普通的js引用沒有實質區別。
我們首先看看“DynLoadTree.Default,DynLoadTree.ashx”的內容:
if(typeof DynLoadTree == "undefined") DynLoadTree={};
DynLoadTree.Default_class = function() {};
Object.extend(DynLoadTree.Default_class.prototype, Object.extend(new AjaxPro.AjaxClass(), { GetChild: function(parentId) {
return this.invoke("GetChild", {"parentId":parentId}, this.GetChild.getArguments().slice(1));
},url: '/ajaxpro/DynLoadTree.Default,DynLoadTree.ashx'}));
DynLoadTree.Default = new DynLoadTree.Default_class();
原來我們DynLoadTree.Default是在這裏定義的,而這個GetChild方法最終是調用“this.invoke("GetChild", {"parentId":parentId}, this.GetChild.getArguments().slice(1));”的,而invoke方法是在“core.ashx”中定義的。在core.ashx中定義了很多Ajax核心的js方法,例如Object.extand實現簡單的繼承(或閱擴展)。在invoke方法裏,首先是new了一個XmlHttp對象,然後重點做了幾件事:
this.xmlHttp.open("POST", this.url, async);
this.xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
this.xmlHttp.setRequestHeader("X-" + AjaxPro.ID + "-Method", method);
this.xmlHttp.send(json);
xmlHttp.open說明了是向哪個服務器url發送請求,是同步請求還是異步請求。接下來就設置Content-Type的http header,然後再將method設置到http header中,以讓服務器端知道要調用什麼方法,最後send出去,同時參數json包含了調用這個方法所需的參數。至此,利用xmlhttp已經將請求發送給服務器了,接下來就等待服務器的反饋結果了(對於同步和異步不同的調用方式,對結果的處理是有區別的)。
但是,爲什麼這樣一個請求給服務器後,服務器就自動調用制定的method呢?如果仔細一點,你可以發現xmlHttp.open裏的this.url到底是什麼?是要調用的頁面的地址麼?實際不是,這個this.url的值是“/ajaxpro/DynLoadTree.Default,DynLoadTree.ashx”。第一次看到這裏的時候,我很詫異,怎麼這個xmlhttp請求也發給一個ashx文件了呢?難道ashx文件不僅僅是用來動態生成js文件的麼?同上,在web.config中已經配置了凡是ashx文件都交由類AjaxPro.AjaxHandlerFactory來處理,要想明白其中的奧祕,還得看看AjaxHandlerFactory裏到底都幹了些什麼。爲此,我用Reflector對AjaxPro.2.dll文件進行反編譯(我的資源裏提供下載),看了AjaxHandlerFactory的代碼才大徹大悟!
原來,在AjaxHandlerFactory的GetHandler方法裏是這麼寫的:
public IHttpHandler GetHandler(HttpContext context, string requestType, string url, string pathTranslated)
{
……
string str2 = requestType;
if (str2 != null)
{
if (!(str2 == "GET"))
{
if (str2 == "POST")
{
if (!(!Utility.Settings.OnlyAllowTypesInList || flag))
{
return null;
}
IAjaxProcessor[] processorArray = new IAjaxProcessor[] { new XmlHttpRequestProcessor(context, type), new IFrameProcessor(context, type) };
for (int i = 0; i < processorArray.Length; i++)
{
if (processorArray[i].CanHandleRequest)
{
if (exception != null)
{
processorArray[i].SerializeObject(new NotSupportedException("This method is either not marked with an AjaxMethod or is not available."));
return null;
}
AjaxMethodAttribute[] customAttributes = (AjaxMethodAttribute[]) processorArray[i].AjaxMethod.GetCustomAttributes(typeof(AjaxMethodAttribute), true);
bool useAsyncProcessing = false;
HttpSessionStateRequirement readWrite = HttpSessionStateRequirement.ReadWrite;
if (Utility.Settings.OldStyle.Contains("sessionStateDefaultNone"))
{
readWrite = HttpSessionStateRequirement.None;
}
if (customAttributes.Length > 0)
{
useAsyncProcessing = customAttributes[0].UseAsyncProcessing;
if (customAttributes[0].RequireSessionState != HttpSessionStateRequirement.UseDefault)
{
readWrite = customAttributes[0].RequireSessionState;
}
}
switch (readWrite)
{
case HttpSessionStateRequirement.ReadWrite:
if (useAsyncProcessing)
{
return new AjaxAsyncHttpHandlerSession(processorArray[i]);
}
return new AjaxSyncHttpHandlerSession(processorArray[i]);
case HttpSessionStateRequirement.Read:
if (useAsyncProcessing)
{
return new AjaxAsyncHttpHandlerSessionReadOnly(processorArray[i]);
}
return new AjaxSyncHttpHandlerSessionReadOnly(processorArray[i]);
case HttpSessionStateRequirement.None:
if (useAsyncProcessing)
{
return new AjaxAsyncHttpHandler(processorArray[i]);
}
return new AjaxSyncHttpHandler(processorArray[i]);
}
if (!useAsyncProcessing)
{
return new AjaxSyncHttpHandlerSession(processorArray[i]);
}
return new AjaxAsyncHttpHandlerSession(processorArray[i]);
}
}
}
}
else
{
switch (fileNameWithoutExtension.ToLower())
{
case "prototype":
return new EmbeddedJavaScriptHandler("prototype");
case "core":
return new EmbeddedJavaScriptHandler("core");
……
default:
return new TypeJavaScriptHandler(type);
}
}
}
return null;
}
它首先對requestType進行判斷,如果是“GET”請求,則說明是html裏對被引用的ashx文件的下載請求,則調用相應的Handler去生成對應的JavaScript內容輸出到客戶端;如果是“POST”請求,則說明是通過XMLHTTP發送過來的,是請求調用服務器端方法的,則返回相應的Handler利用反射機制調用請求的方法。
首先看看“GET”請求,對“GET”請求的處理很簡單,根據不同的文件名返回不同的Handler,對於“core”及“prototype”則返回EmbeddedJavaScriptHandler,對於“DynLoadTree.Default,DynLoadTree.ashx”則返回TypeJavaScriptHandler。在EmbeddedJavaScriptHandler中,構造函數的參數表示要請求的是哪個文件,然後在ProcessRequest函數中提取指定的文件內容並輸出到客戶端,其實這些文件內容都是固定的,且已經放在資源裏的:
internal class EmbeddedJavaScriptHandler : IHttpHandler
{
// Fields
private string fileName; // Methods
internal EmbeddedJavaScriptHandler(string fileName)
{
this.fileName = fileName;
} public void ProcessRequest(HttpContext context)
{
……
string[] strArray = this.fileName.Split(new char[] { ',' });
Assembly executingAssembly = Assembly.GetExecutingAssembly();
for (int i = 0; i < strArray.Length; i++)
{
Stream manifestResourceStream = executingAssembly.GetManifestResourceStream("AjaxPro.2." + strArray[i] + ".js");
if (manifestResourceStream != null)
{
StreamReader reader = new StreamReader(manifestResourceStream);
context.Response.Write(reader.ReadToEnd());
context.Response.Write("/r/n");
reader.Close(); if ((strArray[i] == "prototype") && Utility.Settings.OldStyle.Contains("objectExtendPrototype"))
{
context.Response.Write("/r/nObject.prototype.extend = function(o, override) {/r/n/treturn Object.extend.apply(this, [this, o, override != false]);/r/n}/r/n");
}
}
}
………
}
……
}
對於“DynLoadTree.Default,DynLoadTree.ashx”的請求,則交給TypeJavaScriptHandler處理:
internal class TypeJavaScriptHandler : IHttpHandler, IReadOnlySessionState, IRequiresSessionState
{
// Fields
private Type type;
// Methods
internal TypeJavaScriptHandler(Type type);
public void ProcessRequest(HttpContext context);
// Properties
public bool IsReusable { get; }
}
ProcessRequest會根據Type動態生成JavaScript內容並輸出到客戶端。 對於requestType是“POST”的請求,則返回相應的Handler進行處理。以AjaxSyncHttpHandler爲例:
internal class AjaxSyncHttpHandler : IHttpHandler
{
// Fields
private IAjaxProcessor p; // Methods
internal AjaxSyncHttpHandler(IAjaxProcessor p)
{
this.p = p;
} public void ProcessRequest(HttpContext context)
{
new AjaxProcHelper(this.p).Run();
} // Properties
public bool IsReusable
{
get
{
return false;
}
}
}
其中ProcessRequest方法就就新建一個AjaxProcHelper對象,用該對象的Run方法來處理實質請求。可以簡略看看AjaxProcHelper.Run的代碼:
internal void Run()
{
……
this.p.Context.Response.Expires = 0;
this.p.Context.Response.Cache.SetCacheability(HttpCacheability.NoCache);
this.p.Context.Response.ContentType = this.p.ContentType;
this.p.Context.Response.ContentEncoding = Encoding.UTF8;
……
object[] args = null;
object o = null;
args = this.p.RetreiveParameters();
string key = string.Concat(new object[] { this.p.Type.FullName, "|", this.p.GetType().Name, "|", this.p.AjaxMethod.Name, "|", this.p.GetHashCode() });
if (this.p.Context.Cache[key] != null)
{
this.p.Context.Response.AddHeader("X-AjaxPro-Cache", "server");
this.p.Context.Response.Write(this.p.Context.Cache[key]);
}
else
{
……
if (this.p.AjaxMethod.IsStatic)
{
o = this.p.Type.InvokeMember(this.p.AjaxMethod.Name, BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Static | BindingFlags.IgnoreCase, null, null, args);
}
else
{
……
object obj3 = Activator.CreateInstance(this.p.Type, new object[0]);
o = this.p.AjaxMethod.Invoke(obj3, args);
}
……
if ((o != null) && (o.GetType() == typeof(XmlDocument)))
{
this.p.Context.Response.ContentType = "text/xml";
((XmlDocument) o).Save(this.p.Context.Response.OutputStream);
}
……
}
}
可以清晰的看到,Run中是通過反射機制調用相應的方法,再將結果寫入context輸出到客戶端的。
另外,我們也可以清晰的看到Utility中對RegisterTypeForAjax的幾個重載及實現方式:
public static void RegisterTypeForAjax(Type type);
public static void RegisterTypeForAjax(Type type, Page page);
同時,也可以看看AjaxMethodAttribute的定義(有關Attribute MSDN中有詳細的描述和實例):
[AttributeUsage(AttributeTargets.Method, AllowMultiple=false)]
public class AjaxMethodAttribute : Attribute
{
// Fields
private HttpSessionStateRequirement requireSessionState;
private bool useAsyncProcessing; // Methods
public AjaxMethodAttribute();
public AjaxMethodAttribute(HttpSessionStateRequirement requireSessionState);
[Obsolete("The use of this argument is currently in beta state, please report any problems to [email protected].")]
public AjaxMethodAttribute(bool useAsyncProcessing);
[Obsolete("The recommended alternative is AjaxPro.AjaxServerCacheAttribute.", true)]
public AjaxMethodAttribute(int cacheSeconds);
[Obsolete("The recommended alternative is AjaxPro.AjaxNamespaceAttribute.", true)]
public AjaxMethodAttribute(string methodName);
[Obsolete("The use of this argument is currently in beta state, please report any problems to [email protected].")]
public AjaxMethodAttribute(HttpSessionStateRequirement requireSessionState, bool useAsyncProcessing);
[Obsolete("The recommended alternative is AjaxPro.AjaxServerCacheAttribute.", true)]
public AjaxMethodAttribute(int cacheSeconds, HttpSessionStateRequirement requireSessionState);
[Obsolete("The recommended alternative for methodName is AjaxPro.AjaxNamespaceAttribute.", true)]
public AjaxMethodAttribute(string methodName, HttpSessionStateRequirement requireSessionState);
[Obsolete("The recommended alternative for methodName is AjaxPro.AjaxNamespaceAttribute.", true)]
public AjaxMethodAttribute(string methodName, int cacheSeconds);
[Obsolete("The recommended alternative for methodName is AjaxPro.AjaxNamespaceAttribute.", true)]
public AjaxMethodAttribute(string methodName, int cacheSeconds, HttpSessionStateRequirement requireSessionState); // Properties
internal HttpSessionStateRequirement RequireSessionState { get; }
internal bool UseAsyncProcessing { get; }
}
最後,需要說明的是本文轉載於:http://www.cnblogs.com/cwy173/,本文中提及的只是Ajax機制中的一小部分,例如數據加密、同步異步機制、訪問服務器端的屬性及Session、返回表狀數據及數據轉換等等都未涉及,權當拋磚引玉吧。有很多理解不到位或言辭不嚴謹的地方,切勿全信,僅供參考!